Conventional thermal, flow measuring devices usually use two temperature sensors which are embodied as equally as possible, and which are arranged in (most often pin-shaped) metal housings—so-called stingers—and which are in thermal contact with the medium flowing through a measuring tube or through the pipeline. For industrial application, the two temperature sensors are usually installed in a measuring tube; the temperature sensors can, however, also be installed directly in the pipeline. One of the two temperature sensors is a so-called active temperature sensor, which is heated by means of a heating unit. As the heating unit, either an additional resistance heating is provided, or the temperature sensor itself is a resistance element—e.g. an RTD (Resistance Temperature Device) sensor—which is heated through conversion of electrical power, e.g. through a corresponding variation in the electrical measuring current. The second temperature sensor is a so-called passive temperature sensor; it measures the temperature of the medium.
In a thermal, flow measuring device, the heatable temperature sensor is usually heated in such a way, that a fixed temperature difference arises between the two temperature sensors. Alternatively, it is also known to supply a constant heating power via a regulating/control unit.
If there is no flow in the measuring tube, an amount of heat which is constant in time is then required for maintaining the predetermined temperature difference. If, in contrast, the medium to be measured is in movement, the cooling of the heated temperature sensor is essentially dependent on the mass flow of the medium flowing past. Since the medium is colder than the heated temperature sensor, heat from the heated temperature sensor is transported away by the flowing medium. In order to then maintain the fixed temperature difference between the two temperature sensors in the case of a flowing medium, an increased heating power is required for the heated temperature sensor. The increased heating power is a measure for the mass flow of the medium through the pipeline.
If, in contrast, a constant heating power is fed in, the temperature difference existing between the two temperature sensors as a result of the flow of the medium is lessened. The particular temperature difference is then a measure for the mass flow of the medium through the pipeline or through the measuring tube.
There is, thus, a functional relationship between the heating energy needed for heating the temperature sensor and the mass flow through a pipeline or through a measuring tube. The dependence of the so-called heat transfer coefficient on the mass flow of the medium through the measuring tube (or through the pipeline) is utilized in thermal, flow measuring devices for determining the mass flow. Devices which operate according to this principle are available from the assignee under the name “t-switch”, “t-trend” or “t-mass”.
Until now, mainly RTD-elements with helically wound platinum wires have been applied in thermal, flow measuring devices. In the case of thin film, resistance thermometers (TFRTDs), a meander-shaped platinum layer is conventionally vapor deposited onto a substrate. Over this is applied a glass layer, for protecting the platinum layer. The cross section of thin film, resistance thermometers is rectangular, in contrast to RTD elements having a round cross section. The heat transfer in the resistance element and/or from the resistance element accordingly occurs via two oppositely lying surfaces, which together make up a large part of the total surface of a thin film, resistance thermometer.
The installation of a cuboid-shaped thin film, resistance thermometer into a round, pin-shaped shell is achieved in U.S. Pat. No. 6,971,274 and U.S. Pat. No. 7,197,953 in the following ways. The thin film, resistance thermometer is inserted into a metal spacer with a rectangular recess in such a way, that at least the two oppositely lying large surfaces of the thin film, resistance thermometer have virtually gap-free contact with the surfaces of the spacer lying opposite them. For this, the spacer has a rectangular recess, which is manufactured corresponding to the outer dimensions of the thin film, resistance thermometer. The spacer should tightly hold the thin film, resistance thermometer. In this regard, the spacer and thin film, resistance thermometer virtually form a press fit. The spacer itself and the pin-shaped shell likewise form a press fit. In this way, use of a potting compound or another sort of fill material is unnecessary. The advantage of this construction is good heat transfer between the thin film, resistance thermometer and measured medium on all sides, through the spacer. However, due to the firm fit of the thin film, resistance thermometer and/or due to different coefficients of thermal expansion for the participating materials, mechanical stresses arise in the thin film, resistance thermometer.